PROJECT SUMMARY Apoptosis negatively regulates cellular populations during both physiologic brain growth and the pathologic growth of brain tumors. In the postnatal cerebellum, apoptosis limits the proliferation of cerebellar granule neuron progenitors (CGNPs) and suppresses the formation of medulloblastoma, a tumor of excess neural progenitor proliferation that is the most common malignant brain tumor of children. My project will investigate the mechanisms that regulate apoptosis in CGNPs and apply this understanding to cerebellar hypoplasia and medulloblastoma. CGNPs proliferate in response to Sonic Hedgehog (SHH) signaling in the early postnatal period. Excessive SHH activation in CGNPs causes medulloblastoma, while excessive CGNP apoptosis can produce cerebellar hypoplasia. CGNPs maintain a specialized capacity for rapid apoptosis that is shared by medulloblastoma and mediated by interaction of the pro-apoptotic protein BAX and the anti- apoptotic homolog BCL-xL. It is unknown if specific mechanisms link SHH-driven proliferation and BCL- xL/BAX-regulated apoptosis in CGNPs. I have generated preliminary data suggesting that BCL-xL and SHH activation cooperate to suppress apoptosis in both CGNPs and medulloblastoma. In transgenic mice, genetic deletion of Bcl-xL in the cerebellum results in symptomatic cerebellar hypoplasia, with increased apoptosis occurring in differentiating CGNPs. These findings suggest that in the absence of BCL-xL a redundant anti-apoptotic mechanism prevents cell death in CGNPs undergoing proliferation. This proposal uses the combination of genetic deletion of Bcl-xL and the pharmacological inhibition of SHH with the drug Vismodegib to determine the role of these two mechanisms in controlling CGNP survival and cell death. I will also determine the role of these mechanisms in medulloblastoma, and gain further insight into cell cycle dynamics and transcriptional changes through the use of FACS and Drop-sec. This project will elucidate the regulation of apoptosis in CGNPs and medulloblastoma, provide insight into the pathogenesis of cerebellar hypoplasia, and test the therapeutic potential of combined disruption of SHH and BCL-xL as a novel approach to medulloblastoma therapy. My proposal will give me critical training and experience in independent research, including mouse genetics, analysis of protein interactions, FACS, Drop-seq and bioinformatics, and ethical research conduct, while enhancing my scientific communication skills.